To facilitate the handling, storage and further processing of tobacco, the leaves are threshed by forcing them through "baskets", plates having suitable openings defined by a grid that shreds the leaves so that the pieces can pass through the openings. Threshing reduces the intact leaves to particles, so the leaf fraction may be separated from the sand, stems and other foreign material.
Many variables affect the threshold operation. Among them are the grade of the tobacco, the temperature, the moisture content, the size of the baskets and the rotor speed of the thresher. Variables affecting separation include the velocity of the air, the balance and distribution of the air flow, the ratio of stem to lammina and the number of separation stages.
In practice, threshing is presently carried out commercially under operating conditions that have been determined empirically. There is no scientific way of optimizing the variables of the threshing and separation process to produce an optimum result, namely, a size distribution in the threshed product that yields the largest amount of usable tobacco and minimizes the waste in the form of pieces too small to be processed further for direct use in tobacco products. Tobacco is very costly, and important economic advantages are available from an optimum threshing process.
One of the factors that has hindered the development of an optimum threshing process on a scientific and quantitative basis, particularly the ability to alter the process variables in accordance with the material variables in a manner that yields the best results, has been the lack up to now of any method and apparatus for determining the precise particles size distribution of threshed tobacco. It has, therefore, been impossible to enhance the threshing and separation process, because the process can never be more precise than are the measurements of the results of the process.
It is also desirable to make accurate determinations of the size distributions of cut tobacco for essentially the same reasons as in the case of threshed tobacco, namely, to aid in perfecting the control of the cutting operation for optimum production of usable tobacco in the end product.